Thread Content
During operation of the turbine, the high-speed steam stream ejected from the nozzle cascade enters the rotor cascade, exerting a force on it; the magnitude of this force is related to the velocity of the steam at the exit of the nozzle cascade. Due to the non-uniform distribution of the steam velocity at the outlet of the nozzle cascade in the circumferential direction, the stress conditions on the moving blades undergo periodic changes as they rotate one full cycle. Under the action of alternating steam impact forces, the moving blades vibrate. The excitation forces that cause vibration in the rotating blades are divided into high-frequency excitation forces and low-frequency excitation forces. The high-frequency excitation force is an alternating force generated by the friction between the steam and the wall surface of the nozzle cascade, which causes the steam velocity to be unevenly distributed in the circumferential direction; its frequency is equal to the product of the equivalent number of nozzles in that stage and the rotor rotation frequency. The low-frequency excitation force is due to the grouping of nozzles in the regulation stage ； Due to machining errors in the horizontal joint surfaces of the upper and lower partitions, the profile of the nozzle blades may be misaligned, or the width of the nozzle flow channel at the joint surface may differ from that of other flow channels ； Errors in the grid pitch of individual nozzle vanes are caused by structural factors such as changes in the flow channel geometry. As the moving blade completes one rotation, its stress condition changes K times, with a frequency that is K times that of the rotor’s rotation frequency. When the natural frequency of the moving blade equals the frequency of the exciting force, resonance occurs and the dynamic stress reaches its maximum value. Under non-resonant conditions, the dynamic stress is greatest when the flow rate in a single nozzle channel is at its maximum.